On the Internet, users actively access WWW servers from Web browsers, which are running on client terminals, to browse document data, image data, and the like on the client terminal side via the Web browsers. Each WWW server saves a document called a home page, which is prepared by describing information to be disclosed in HTML. The Web browser on the client terminal side accesses such home page and displays it on the display screen of a display device of the client terminal. Also, when the user designates a link in the home page displayed on the display device, the Web browser on the client terminal side can acquire required information by tracing a link destination indicated by the link.
Furthermore, as a method of downloading files managed by the WWW server, a method called “File Transfer Protocol” (to be abbreviated as “FTP” hereinafter) is known. The FTP is a scheme for transferring the contents of a file on the WWW server to the client computer at a time via a network.
One of techniques for transferring the whole image file at a time is to control the order of transmission data on the server side (for example, see Japanese Patent Laid-Open No. 2000-50261). In this case, some regions are set as important regions on the server side, and pieces of information in these regions are encoded in order before the remaining pieces of region information. The client can determine on an early stage of transmission whether the current image is necessary or unnecessary, and can interrupt reception of the data in the middle of the operation.
As a protocol for fragmentarily accessing and displaying an image data file, Flashpix/IIP is known. This Internet imaging protocol (IIP) is optimal to the image data file format “Flashpix”, and makes partial access to image data for respective tiles of Flashpix. Some conventional techniques that use this IIP protocol have been disclosed (for example, see Japanese Patent Laid-Open No. 2002-49514).
With any protocol, the client terminal cannot transmit to the server side a request to limit the size of response data.
As of June, 2003, JPEG2000 image coding system—Part 9: Interactivity tools, APIs and Protocols (to be abbreviated as JPIP hereinafter) has been examined as a protocol for fragmentarily accessing and displaying a JPEG2000 file. JPEG2000 and JPIP enable access to a partial region of an image and selection of the resolution, similar to Flashpix and IIP. Also, a change of the image quality, color/monochrome switching, and setting of the size limitation of response data can be easily achieved in accordance with client requests. When JPEG2000 image data is fragmentarily transmitted using JPIP, the client must cache received fragmentary encoded data so as to decode the image. This is because encoded data of each JPEG2000 layer/level is difference data from data one layer/level lower than that layer/level.
According to the technique of setting a predetermined region as an important region on the server side, like Japanese Patent Laid-Open No. 2000-50261, the server must set and encode again an important region every time the server receives a request. Thus, the server cannot cope with various requests (e.g., a request of a partial region of an image and requests to switch the resolution, image quality, and color/monochrome) from the client using interactive image communication protocols such as IIP and JPIP.
When the response data size receivable or cacheable on the client side is limited, the client may stop reception of data by the limited size. If the server transmits data in a preset transmission order without considering the size limitation of reception data on the client side, the client may display only some of requested regions at a high definition upon decoding the received data, and may not display the remaining data. Alternatively, the client may display only some of display regions at a very high definition, and display adjacent regions unnaturally at a very low resolution.